In recent years, white color light emission diodes have become widely used as backlights of a portable apparatus having a liquid display device (LCD), like mobile phones or digital cameras. A driving voltage of a white color light emission diode is 3.4 Volts to 3.7 Volts, whereas a driving voltage of a lithium-ion cell, which is frequently used as the power supply of a portable apparatus, is 3.5 Volts to 4.2 Volts. Since the driving voltage of the white color light emission diode is close to the driving voltage of the lithium-ion cell acting as the power supply, in order to drive the white color light emission diode for a long time period, it is necessary to use a step-up circuit (booster) like a charge pump circuit to boost the voltage of the power supply.
For example, Japanese Laid Open Patent Application No. 2006-187187 (below, referred to as “reference 1”) discloses a configuration of a power supply device in the related art able to appropriately set a step-up ratio (or boost ratio) of a charge pump circuit so as to supply a constant current to a light emission diode.
FIG. 7 is diagram illustrating a configuration of a power supply device in the related art as disclosed in reference 1.
In a power supply device 100 as shown in FIG. 7, a light emission diode 110 acting as a load is driven by a constant current supplied by a constant-current circuit 101. A monitoring circuit 102 monitors a voltage Vled of a connection portion between the constant-current circuit 101 and the light emission diode 110, and the monitoring circuit 102 has a voltage source (not illustrated) which generates a threshold voltage Vth following variation of a voltage enabling constant operations of the constant-current circuit 101.
The monitoring circuit 102 compares the threshold voltage Vth generated by the voltage source with the voltage Vled, and when a state in which the voltage Vled is lower than the threshold voltage Vth continues for a certain time period, a comparison result signal Ss is output to a controller 103.
The controller 103 increases a step-up ratio of the charge pump circuit 104 based on the comparison result signal Ss from the monitoring circuit 102; thus the voltage Vled is adjusted to have a value enabling constant operations of the constant-current circuit 101, and this allows the light emission diode 110 to be driven by a constant current.
In FIG. 7, the voltage of the battery 111 acting as a power supply decreases gradually along with usage, and hence, in FIG. 7, the step-up ratio of the charge pump circuit 104 is allowed to be variable from a low value to a high value. For this reason, when the state in which the comparison result signal Ss from the monitoring circuit 102 is at a low level, that is, the voltage Vled is lower than the threshold voltage Vth, continues for a certain time period, the controller 103 increases the step-up ratio of the charge pump circuit 104 by one level.
For example, when the charge pump circuit 104 is operating with the step-up ratio of one, if the comparison result signal Ss from the monitoring circuit 102 becomes the low level, the controller 103 increases the step-up ratio of the charge pump circuit 104 to 1.5. Similarly, when the charge pump circuit 104 is operating with the step-up ratio of 1.5, when the comparison result signal Ss from the monitoring circuit 102 becomes the low level, the controller 103 increases the step-up ratio of the charge pump circuit 104 to 2.
However, in the above method, once the voltage of the battery decreases, the step-up ratio of the charge pump circuit 104 is increased. If the voltage of the battery increases again later, although ideally the step-up ratio of the charge pump circuit 104 should be decreased to maintain the voltage Vled at an optimum value, in the above method, the step-up ratio of the charge pump circuit 104 remains at the current high value, and this causes the light emission diode 110 to be over-driven by a large current.
Since the driving efficiency is low when the step-up ratio of the charge pump circuit 104 is large, it is desirable that the step-up ratio of the charge pump circuit 104 be as small as possible. However, in the related art, sometimes, the light emission diode 110, serving as a load, is driven with a large step-up ratio of the charge pump circuit 104, and in this case, the driving efficiency of the device is low.
For example, suppose the driving current of the light emission diode 110 can have an arbitrary value. In the period when the light emission diode 110 is driven to emit light by a large current, since the voltage of the battery decreases because of the large current, it is necessary to increase the voltage Vled, and hence, the threshold voltage Vth is set to be a high value. Consequently, the step-up ratio of the charge pump circuit 104 becomes insufficient; due to this, the step-up ratio of the charge pump circuit 104 is increased to drive the light emission diode 110. Afterward, when the driving current of the light emission diode 110 is decreased, the load current also decreases, and thus the voltage of the battery increases. Furthermore, since the threshold voltage Vth also decreases, a small step-up ratio of the charge pump circuit 104 is sufficient to drive the light emission diode 110. However, in the device shown in FIG. 7, the light emission diode 110 is still driven with the step-up ratio of the charge pump circuit 104 remaining a high value, and the driving efficiency of the device is significantly low.
On the other hand, if the device is configured so that the step-up ratio of the charge pump circuit 104 can be easily returned to a low value, when a motor requiring a large current is used in the portable apparatus, it ends up that the step-up ratio of the charge pump circuit 104 changes frequently, and brightness of the light emission diode 110 may fluctuate. To avoid this problem, conditions required for increasing the step-up ratio of the charge pump circuit 104 and conditions required for decreasing the step-up ratio of the charge pump circuit 104 are allowed to be changed to avoid frequent switching of the step-up ratio of the charge pump circuit 104.
With the conditions required for increasing or decreasing the step-up ratio of the charge pump circuit 104 being able to be changed, when changing the brightness of the light emission diode 110, in most cases, the brightness of the light emission diode 110 is lowered sequentially from a high value to a low value, and the brightness adjustment is finished when the brightness of the light emission diode 110 becomes a preset value. Since the brightness of the light emission diode 110 is set to be high at the beginning, a large current is required to drive the light emission diode 110, and the step-up ratio of the charge pump circuit 104 is set to be large. However, at the time the brightness adjustment is finished, even though a small step-up ratio of the charge pump circuit 104 is sufficient, as the conditions required for increasing the step-up ratio of the charge pump circuit 104 are different from the conditions required for decreasing the step-up ratio of the charge pump circuit 104, the step-up ratio of the charge pump circuit 104 remains to be a high value.